Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature second stage array, usually operating in the range of 4 degrees to 25 degrees Kelvin, is the primary pumping surface. This surface is surrounded by a high temperature cylinder, usually operated in the temperature range of 50 degrees to 130 degrees Kelvin, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing. The housing is closed except at a location near frontal array. The radiation shield is between the primary pumping surface and the chamber to be evacuated. This higher temperature, first stage, frontal array serves as a pumping site for higher boiling point gases such as water vapor.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that frontal array and pass into the volume within the radiation shield and condense on the second stage array. A surface coated with an adsorbent (such as charcoal or a molecular sieve) operating at or below the temperature of a second stage array may also be provided in this volume. The adsorbent removes or captures the very low boiling point gases. With the gases condensed or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In systems cooled by closed cycle coolers, the cooler is typically a two stage refrigerator having a cold finger which extends through the radiation shield. The cold end of the second, coldest stage of the refrigerator is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at the coldest end of the second stage of the cold finger. This cryopanel may be a simple metal plate, a cup or a cylindrical array of metal baffles arranged around and connected to the second stage heat sink. This second stage cryopanel may also support low temperature adsorbent.
The radiation shield is connected to a heat sink, or heat station at the coldest end of the first stage of the refrigerator. The shield surrounds the first stage cryopanel in such a way as to protect it from radiant heat.